Gas shock absorbers are well known in the art. These shock absorbers include a primary cylinder which is connected to a secondary cylinder. The primary cylinder contains a substantially incompressible fluid such as oil, and the secondary cylinder contains a compressible fluid such as a gas. In use, the gas is alternatingly compressed and expanded by the action of the primary cylinder, thereby providing a desired level of damping response. One of the factors which determines the characteristics of the damping is the size of the passageway between the primary and secondary cylinders. A large passageway allows a fast interaction between the cylinders. A small passageway only allows a slow interaction.
The size of the passageway may be fixed or adjustable. In many situations, a fixed size is useful. In other situations, an adjustable passageway is preferred. The size of the passageway is effectively adjusted using a valve. The most common valves are needle valves but other types of valves are sometimes used.
For example, U.S. Pat. No. 4,153,237 shows a hydrapneumatic suspension unit for a vehicle having first and second chambers separated by a valve assembly for controlling the flow of liquid between them. The first chamber is filled with a liquid and includes a slidably  mounted piston connected by a linkage to the road wheel. The second chamber has a first compartment containing the liquid and a second compartment containing a compressible medium separated by a floating piston. The valve assembly includes a needle damping valve 37, a low-restriction check valve for permitting one-way flow of liquid from the first to the second chamber, and a damping bypass valve for enabling quick return of liquid from the second chamber to the first chamber.
U.S. Pat. No. 4,311,302 illustrates a shock absorber device, having a piston rod unit that is axially displaceable through a cylinder member from one end toward the other end. The cylinder forms a variable volume working chamber containing a first fluid. A spring chamber is in communication with the working chamber so that the first fluid can flow between the two chambers. A pressurized second fluid is contained within the spring chamber separate from the first fluid and acts on the first fluid to bias the piston rod unit out of the working chamber. When the piston rod unit is moved into the working chamber and approaches an inner end position, it forms at least in part a damping chamber which decelerates the movement of the piston rod unit. The passageway between the two chambers is fixed. No external adjustment of the rate of damping is available.
U.S. Pat. No. 4,620,694 discloses a hydraulic suspension system for a motorcycle having an adjustable shock absorber unit located between the rear wheel and main frame. The first of four adjustments is an externally accessible filler-valve enabling the rider to adjust gas pressure in one chamber to vary spring rate. A second adjustment is a valve for varying the rate of hydraulic fluid flow from a main chamber through a main piston to a third chamber to obtain the desired rate of upper wheel travel. A third adjustment is an adjustable pressure relief valve for limiting differential pressure between fluid in the main chamber and in the third chamber under conditions of severe impact. A fourth adjustment is a valve for varying the fluid flow from a fourth chamber to the third chamber during extension.
U.S. Pat. No. 4,921,224 illustrates a hydro-pneumatic suspension system for a car having a main housing with a damping device and a sub housing having a metal bellows. A high-pressure compressed gas is sealed inside the bellows. A passageway connects the main housing to the subhousing. No means is provided for adjusting the size of the passageway. 
U.S. Pat. No. 4,958,706 describes an adjustable shock absorber having a needle type adjusting screw in the flow path between two chambers for altering the characteristics of the flow path.
U.S. Pat. No. 5,351,790 depicts a hydraulically adjustable suspension apparatus for an automobile which has a proportional type of solenoid operated valve assembly for controlling the pressure or flow rate of a pressurized fluid introduced into a cylinder tube through a fluid passage.
U.S. Pat. No. 4,732,244 shows a hydraulic shock damper assembly for the rear wheel of a rally-cross motorcycle and other vehicles. The hydraulic cylinder is connected through a flow channel to a dashpot having a partition element enclosing a pressurized fluid such as nitrogen. A plurality of identical apertures 41 are provided through a disk 40 for adjusting the total aperture between the hydraulic cylinder and dashpot. A control plate 43 is rotated by a knob 45 to adjust the number of apertures in the passageway.
Under harsh conditions such as encountered in the operation of snowmobiles and all-terrain vehicles, needle valves have limited utility because they tend to break. The valve of U.S. Pat. No. 4,732,244 is an example of a valve for use in harsh conditions that is not a needle valve. However, the control plate used is relatively thin. A valve for adjusting a gas shock absorber that is durable would be a useful improvement. 